This invention relates to a hydrocarbon conversion process. The invention more specifically relates to the production of alkylaromatic hydrocarbons by the reaction of an acyclic olefinic hydrocarbon with an aromatic substrate hydrocarbon.
The alkylation of aromatic substrates with olefins to produce monoalkyl aromatics is a well developed art which is practiced commercially in large industrial units. One commercial application of this process is the alkylation of benzene with ethylene to produce ethylbenzene which is subsequently used to produce styrene. Another application is the alkylation of benzene with propylene to form cumene (isopropylbenzene) which is subsequently used in the production of phenol and acetone. Those skilled in the art are therefore familiar with the general design and operation of such alkylation processes.
The performances of alkylation processes for producing monoalkyl aromatics are influenced by the stability and activity of the solid catalyst at the operating conditions of the process. For example, as the molar ratio of aromatic substrate per olefin increases, currently available catalysts typically exhibit an improved selectivity to the monoalkyl aromatic. But even at a high molar ratio of aromatic substrate per olefin, several polyalkyl aromatic by-products such as dialkyl aromatics and trialkyl aromatics accompany monoalkyl aromatic production.
Although the formation of dialkyl and trialkyl aromatics might, at first glance, be viewed as by-products that represent a reduction in the efficient use of the olefin, in fact each can be readily transalkylated with the aromatic substrate using a transalkylation catalyst to produce the monoalkyl aromatic. So-called combination processes combine an alkylation zone with a transalkylation zone in order to maximize monoalkyl aromatic production.
One disadvantage of combination processes is that separate reaction zones for alkylation and for transalkylation duplicate costly equipment. Each reaction zone requires what amounts to its own reaction train, including separate and independent reaction vessels, heaters, heat exchangers, piping, and instrumentation.
Another disadvantage of combination processes is the great expense associated with recovering and recycling unreacted aromatic substrate from the effluents of the alkylation and transalkylation reaction zones. Alkylation reaction zones generally operate at a molar ratio of aromatic substrate per alkylation agent that is at least 1:1 in order to help ensure a high selectivity toward the monoalkyl aromatic. Transalkylation reaction zones generally operate at a molar ratio of aromatic per dialkyl aromatic that is much greater than the stoichiometric ratio of 1:1 in order to help ensure a high conversion of the dialkyl aromatic to the monoalkyl aromatic. Consequently, the alkylation and transalkylation reaction zone effluents contain a significant quantity of unreacted aromatic substrate, which must be removed in order to obtain the monoalkyl aromatic product and which must be recycled in order to ensure the efficient use of the aromatic substrate.
Prior art combination processes lessen the great expense incurred in removing and recycling the unreacted aromatic substrate contained in the alkylation and transalkylation reaction zone effluents by three methods. One method is to pass the alkylation effluent stream and the transalkylation effluent stream to a single, common product recovery facility, in which the same distillation columns remove unreacted aromatic from both effluent streams and recycle unreacted aromatic substrate to both reaction zones. In this method, the respective flows through alkylation and transalkylation can be considered to be in parallel. Incidentally, a no less important function of these distillation columns in the prior art is the removal of polyalkyl aromatics other than dialkyl and trialkyl aromatics and of other heavy alkylation and transalkylation by-products such as diphenylalkanes, which are collectively referred to herein as heavies. Although sharing common product equipment in this manner reduces the capital expense of a combination process, the energy requirements for vaporizing and condensing the aromatic substrate from the effluent streams remains undiminished.
A second method is to pass the entire transalkylation effluent stream to the alkylation zone and then to pass the alkylation effluent stream to the product recovery facility. In this method, the flow through alkylation and transalkylation can be considered as being in series, with transalkylation upstream of alkylation. This arrangement is sometimes referred to as a xe2x80x9ccascadedxe2x80x9d flow scheme, with transalkylation leading alkylation. The advantage of this method is that unreacted aromatic substrate in the transalkylation effluent stream is directly used in alkylation without expending energy to separate the aromatic substrate from monoalkyl and polyalkyl aromatics. However, an upset condition or any other disruption in the operation of the transalkylation zone propagates directly to the alkylation zone, which can disrupt or adversely affect alkylation reactions. Moreover, even if the transalkylation zone is operating at optimum transalkylation conditions, its effluent may not be an optimum feed stream for the alkylation zone.
A typical scenario helps illustrate the susceptibility to transalkylation upsets of alkylation zones in a cascaded flow scheme with transalkylation leading alkylation. It is well known that the performance of transalkylation catalysts can be affected by the concentration of water in the transalkylation reactor. An unexpected ingress of an excessive amount of water into the transalkylation reactor can cause the conversion of polyalkyl aromatics to monoalkyl aromatics to drop precipitously, say from 70% to 50%. When this occurs in a commercial combination process, levels of polyalkyl aromatics begin to accumulate within the product recovery facility, and in response operators increase the flow rate of polyalkyl aromatics to the transalkylation reactor by 40%. In a cascaded flow scheme with transalkylation leading alkylation, this necessarily increases the flow rate of transalkylation effluent, and especially of polyalkyl aromatics, to the alkylation reactor by 40%. There, the polyalkyl aromatics tend to be further alkylated by olefin, which produces even more highly alkylated polyalkyl aromatics that must be converted in transalkylation. Thus, passing transalkylation effluent to alkylation propagates the initial upset from transalkylation to alkylation and destabilizes the entire combination process. This compounding and prolongation of the initial disturbance can necessitate reducing alkylation throughput and lead to significant economic losses.
A third method is to pass the entire alkylation effluent stream to the transalkylation zone and then to pass the transalkylation effluent stream to the product recovery facility. Like the second method, the flow through alkylation and transalkylation can be considered as being in series, but in this method alkylation is upstream of transalkylation. This arrangement is sometimes referred to as a xe2x80x9ccascadedxe2x80x9d flow scheme, with alkylation leading transalkylation. Although this method does not expend energy separating unreacted aromatic substrate from the alkylation effluent stream, passing alkylation effluent to transalkylation significantly decreases the yield of the desired monoalkyl aromatic product.
Thus, the high utilities expenses of combination processes as well as the costly duplication of reaction zones has given impetus to research with a goal of minimizing energy requirements and of integrating the alkylation and transalkylation zones efficiently and economically.
This invention is an economical and efficient combination process for producing an alkyl aromatic by alkylation and by transalkylation. In this invention, one portion of the transalkylation zone effluent stream passes to the alkylation zone, another portion of the transalkylation zone effluent stream passes to the product recovery zone, and the alkylation zone effluent stream also passes to the product recovery zone. Thus, the second portion, but not the first portion, of the transalkylation effluent stream bypasses the alkylation zone. The primary advantage of this invention is that it minimizes the flow of unreacted aromatic substrate to the product recovery zone. For example, in comparison with prior art processes that pass each of the entire transalkylation effluent stream and the entire alkylation effluent stream in parallel to the product recovery zone, this invention decreases the flow of unreacted aromatic substrate to the product recovery zone for a given ratio of aromatic substrate per olefin in the alkylation zone.
Another important advantage of this invention is apparent when this invention is compared to prior art processes that pass the entire transalkylation effluent to the alkylation zone and then pass the alkylation effluent stream to the product recovery zone. This invention allows the alkylation zone to operate with an additional degree of freedom for a given flow rate of unreacted aromatic substrate to the product recovery zone. This additional degree of freedom allows the feed to the transalkylation zone to be adjusted over a wide range of flow rates as necessary to achieve the required conversion of polyalkyl aromatic to monoalkyl aromatic, regardless of the alkylation feed rate or without having an adverse impact on alkylation conditions. For example, this invention avoids the need to increase the alkylation feed rate in response to an unexpected upward spike in the water concentration in the transalkylation reaction zone, even if the transalkylation feed rate is increased. Using this invention, transalkylation effluent in excess of the desired alkylation feed rate can bypass the alkylation reaction zone and pass directly to the product recovery facility. Thus, this invention helps to mitigate, rather than to aggravate, upsets in transalkylation and improves the profitability of combination processes.
The present invention minimizes the flow of aromatic substrate to the product recovery facility of a commercial combination process over a broad range of operating conditions. At these same conditions, the prior art combination processes are incapable of minimizing the flow of aromatic substrate to the product recovery facility. The minimum flow rate of aromatic substrate to the product recovery facility is apparent from an x-y chart in which the y-axis is the flow rate of aromatic substrate to the product recovery facility and the x-axis is the molar ratio of aromatic substrate per alkylating agent in the alkylation zone. Although the exact shape of the curve plotted on such a chart depends on the operating conditions for the alkylation and transalkylation zones, a typical curve that is representative of a the curves for a broad range of commercial operating conditions has a characteristic concave-upward shape. There is an optimum molar ratio for which the flow rate of the aromatic substrate is a minimum. At molar ratios greater than the optimum, the flow rate of aromatic substrate is greater than the minimum, because the aromatic substrate content in the alkylation effluent is high. At molar ratios less than the optimum, a high amount of aromatic substrate is present in the transalkylation effluent, since a large amount of aromatic substrate must be supplied to transalkylation in order to compensate for polyalkylation in alkylation, and so the flow rate of aromatic substrate to the product recovery facility is greater than the minimum. Unlike the prior art combination processes, the present invention provides the flexibility to operate at this optimum molar ratio over a wide range of operating conditions.
It is now recognized that the curve on the x-y chart described in the previous paragraph itself is a function of the fraction of the transalkylation effluent that passes directly to the product recovery facility. For example, when the entire transalkylation effluent passes directly to the product recovery facility, the curve is shifted upward and towards the right on the x-y chart, so that the minimum flow rate of aromatic substrate increases and the optimum molar ratio increases also. This is the position of the curve on the x-y chart for the prior art processes where the transalkylation and alkylation effluents flow in parallel to the product recovery facility. However, for such a process the minimum flow rate of aromatic substrate to the product recovery facility is so high that the process is not economical because the capital and operating costs associated with recycling aromatic substrate are prohibitive. On the other hand, when none of the transalkylation effluent passes directly to the product recovery facility, the curve is shifted downward and towards the left on the x-y chart, so that the minimum flow rate of aromatic substrate and the optimum molar ratio both decrease. The curve on the x-y chart is in this position for the cascaded prior art processes where transalkylation leads alkylation and the entire transalkylation effluent flows in series to the alkylation zone. However, for such a process the optimum molar ratio is so low (e.g., below 1:1), that it is impractical to use the combination process for monoalkylation, since alkylation at extremely low molar ratios produces an unacceptable yield of polyalkyl aromatics byproducts.
The present invention solves the dilemma that is presented by the unacceptable alternatives of the prior art processes. The present invention passes one portion of the transalkylation effluent stream to the alkylation zone and another portion of the transalkylation effluent stream to the product recovery facilities. Therefore, the present invention is capable of operating at minimum flow rates of aromatic substrate to the product recovery facilities that are economical and at optimum molar ratios of aromatic substrate per alkylating agent in alkylation that are practical and suitable for monoalkylation. In addition, the present invention provides an extra degree of operating flexibility that allows the feed rate to the transalkylation zone to be adjusted independently of the feed rate to the alkylation zone. In the event of a water upset or other disturbance in transalkylation, the combination process of this invention can be readily adjusted to rapidly reestablish normal operating conditions.
The costs associated with recycling the alkylation substrate can be decreased by diverting to the transalkylation reaction zone some or all of the alkylation substrate that passes directly to the alkylation reaction zone in some prior art processes. Because diverting this benzene to the transalkylation reaction zone increases conversion of polyalkyl aromatics in the transalkylation reaction zone, the alkylation reaction zone may be operated with less alkylation substrate being passed directly to the alkylation reaction zone. Thus, less excess alkylation substrate is present in the alkylation effluent stream and, therefore, less capital and utilities need to be spent to recover the desired alkylaromatic product from the excess alkylation substrate in the alkylation effluent stream. Consequently, this invention can be operated in a manner that vaporizes, condenses, and recycles a decreased quantity not only of polyalkyl aromatics but also of excess alkylation substrate. Additional cost savings may be attainable with this invention by consolidating the alkylation and transalkylation reaction zones into a single reactor vessel, and by eliminating in whole or in part recycling of the alkylation effluent stream, if any, to the alkylation reaction zone. Thus, in summary, a combination process that uses this invention can operate with significantly lower capital and utility costs compared to a prior art combination process.
Combination processes that will benefit most from this invention include those in which passing the transalkylation reactor effluent to the alkylation reaction zone does not have significant adverse effects on the production of monoalkyl aromatic in the alkylation zone or on the deactivation rate of the alkylation catalyst. For this reason, this invention is particularly applicable to combination processes that use beta zeolite as the alkylation catalyst, because at alkylation conditions beta zeolite produces nearly the equilibrium amount of monoalkyl aromatic and because, surprisingly, beta zeolite is not rapidly deactivated by polyalkyl aromatics in the alkylation feed. This invention is also particularly applicable to those combination processes that benefit from operation at a relatively high molar ratio of phenyl groups per alkyl group in the transalkylation reaction zone and a relatively low molar ratio of aromatic per alkyl group in the alkylation reaction zone.
A broad objective of this invention is to provide an improved combination process for the production of alkyl aromatics that minimizes the capital and operating expenses associated with recycling aromatic substrate feedstock. Another objective is to provide a combination process for the production of alkyl aromatics in which, when upsets in the transalkylation zone occur, such upsets are prevented from propagating to the alkylation zone, stable operation of the process is maintained, and normal operation of the process is reestablished expeditiously.
In a broad embodiment, this invention is a process for producing alkyl aromatic hydrocarbons. A first transalkylation feed stream comprising an aromatic substrate hydrocarbon passes to a transalkylation reaction zone. A second transalkylation feed stream comprising a first alkyl aromatic hydrocarbon having more than one alkyl group passes to the transalkylation reaction zone. In the transalkylation zone, the aromatic substrate hydrocarbon transalkylates with the first alkyl aromatic hydrocarbon in the presence of a first solid catalyst. The transalkylation reaction produces a second alkyl aromatic hydrocarbon having at least one more alkyl group than the aromatic substrate hydrocarbon. A transalkylation effluent stream comprising the aromatic substrate hydrocarbon and the second alkyl aromatic hydrocarbon is recovered from the transalkylation reaction zone. A first alkylation feed stream comprising an alkylating agent passes to an alkylation reaction zone. A first aliquot portion of the transalkylation effluent stream passes to the alkylation reaction zone. In the alkylation zone, the aromatic substrate hydrocarbon alkylates with the alkylating agent in the presence of a second solid catalyst to produce an alkylation effluent stream comprising the second alkyl aromatic hydrocarbon. At least a portion of the alkylation effluent stream and a second aliquot portion of the transalkylation effluent stream pass to a product separation zone. In the product separation zone, the entering compounds are separated into a product stream comprising the second alkyl aromatic hydrocarbon, a first recycle stream comprising the aromatic substrate, and a second recycle stream comprising the first alkyl aromatic hydrocarbon. At least a portion of the first transalkylation feed stream is formed from at least a portion of the first recycle stream. At least a portion of the second transalkylation feed stream is formed from at least a portion of the second recycle stream.
Other objectives and embodiments of this invention are disclosed in the detailed description.
Prior art alkylation processes are well described in the literature.
U.S. Pat. No. 4,051,191 describes catalysts, reaction conditions, and a separation method for the recovery of cumene that uses a rectification zone and a two-column fractionation train.
U.S. Pat. Nos. 4,695,665 and 4,587,370 are particularly directed to the separation of products and the recovery of recycle streams from processes for the alkylation of aromatic hydrocarbons, and U.S. Pat. No. 4,695,665 discloses the use of a flash drum in combination with an effluent rectifier to recover unreacted feed components.
U.S. Pat. No. 4,891,458 describes the use of beta zeolite for the alkylation of aromatic hydrocarbons with alkenes to produce alkyl aromatics. U.S. Pat. No. 4,891,458 also discloses that transalkylation can occur in an alkylation reactor, and that additional monoalkyl aromatic hydrocarbons can be produced in an alkylation reactor by recycling polyalkyl aromatic hydrocarbons to the alkylation reactor to undergo transalkylation.
U.S. Pat. No. 4,922,053 describes a process for alkylating benzene with ethylene in a multibed reactor wherein polyethylbenzenes are recycled to the first alkylation bed and also to one or more of the other alkylation beds in order to increase ethylbenzene yield.
U.S. Pat. No. 5,030,786 discloses an alkylation process wherein the feed stream is dehydrated to enhance the performance of beta or Y zeolites in the alkylation process.
U.S. Pat. No. 5,336,821 describes the use of beta zeolite for the alkylation of aromatic hydrocarbons in a process that is improved by an indirect heat exchanger to recover the heat of reaction. In one embodiment, the alkylation reactor effluent passes through the heat exchanger and is recycled to the alkylation reactor.
Prior art transalkylation processes are well described in the literature. U.S. Pat. No. 4,083,886 describes a process for the transalkylation of the alkyl aromatic hydrocarbons that uses a zeolitic catalyst. U.S. Pat. No. 4,891,458 describes the use of beta zeolite for the transalkylation of aromatic hydrocarbons with polyalkyl aromatic hydrocarbons. European Patent Application EP 0 733 608 A1 discloses the use of an alumina silicate catalyst having an average crystal size of less than about 0.5 microns for the transalkylation of polyalkyl benzenes with benzene in a reaction zone with an alkylating agent such as ethylene.
Combination processes that produce alkyl aromatic products by using an alkylation reaction zone and a transalkylation reaction zone are also well known.
U.S. Pat. No. 4,008,290 describes a combination process in which the alkylation effluent and the transalkylation effluent are passed to a common separation zone, which separates the two effluents into product, by-product, and recycle streams including a recycle benzene stream. A portion of the alkylation effluent is recycled to the alkylation reaction zone in order to decrease the portion of the recycle benzene stream that is recycled to the alkylation reaction zone.
U.S. Pat. No. 5,003,119 describes a combination process for producing monoalkyl aromatics in which the alkylation effluent passes to the transalkylation reaction zone, and the transalkylation effluent passes to a separation zone. U.S. Pat. No. 5,003,119 also describes passing dialkyl aromatics to the alkylation reaction zone.
U.S. Pat. No. 5,177,285 discloses an alkylation process that is improved by maintaining the feed to the alkylation zone in a relatively wet condition and the feed to the transalkylation zone in a relatively dry condition. The process operates with a relatively pure ethylene feed as an alkylating agent with a large excess of benzene.
U.S. Pat. No. 5,723,710 describes a surface-modified zeolite beta which exhibits stability and long catalyst life when used in alkylation and transalkylation of aromatic compounds. The teachings of U.S. Pat. No. 5,723,710 are incorporated herein by reference.
U.S. Pat. No. 5,902,917 describes a process for producing alkylaromatics, especially ethylbenzene and cumene, wherein a feedstock is first fed to a transalkylation zone and the entire effluent from the transalkylation zone is then cascaded directly into an alkylation zone along with an olefin alkylating agent, especially ethylene or propylene.
U.S. Pat. No. 5,998,684 describes a process for producing alkylaromatics that operates with an alkylation zone and a transalkylation zone, where the transalkylation zone and the alkylation zone are arranged for series flow and the transalkylation zone effluent is passed with an aromatic containing feed and the olefinic feed, which is preferably propylene or ethylene, to the alkylation zone.